In the past, the penetration index was defined based on the assumption that thermal sensitivity in the bitumen used in road construction was zero. Bitumen is a thermoplastic material, meaning that it becomes pliable or moldable above a specific temperature and returns to a solid state upon cooling.
Penetration-grade bitumen is mainly used in road surfacing. Bitumen with lower penetration grade is used in the regions with warm climate while higher penetration grade is used in colder weather.